As lithography technology is improving and features get smaller and smaller, further improvements in precision, resolution and accuracy are required. It is important to realize that the CD budget can no longer be treated as a separated control measure. For example, in the sub 45 nm nodes, using the Double Patterning Technology (DPT), overlay errors become part of the overall CD budget.
As the placement error is one of the errors which have impact on the CD, it is important to be able to measure the placement error at the feature resolution level. Since some of the CD budget is due to the mask, it is important to be able to measure the placement error between different locations on the mask (or between masks).